1. Field of the Invention
The present invention relates to a carrier collecting device for collecting a carrier deposited on a photosensitive member in development for use in an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer for performing development by using a two-component developer.
2. Description of the Related Art
In an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer, a two-component developer having a toner and a carrier is conventionally widely used in performing development. A developing unit using such a two-component developer is so designed as to deposit only the toner onto a photosensitive member and recover the carrier not deposited onto the photosensitive member. However, the carrier is not completely recovered but some of the carrier is deposited onto the photosensitive member. If the carrier is deposited onto the photosensitive member, the toner present in the periphery of the carrier on the photosensitive member cannot enough come into contact with a transfer paper, causing the generation of white spots or the like.
To prevent this defect, there has conventionally been proposed a device for collecting the carrier deposited onto the photosensitive member. For example, there is proposed in Japanese Patent Laid-open No. Sho 62-262074 a device for collecting the carrier by forming an electric field between a collecting member and a photosensitive member. Further, there is proposed in Japanese Utility Model Publication No. Sho 53-32599 a device for collecting the carrier by a magnetic attraction force.
In recent years, a copying machine or a printer using electrophotography has a tendency that a gap between a photosensitive member and a developing roller is reduced to perform development with a strong development field in order to prevent a line of electric force from turning about from an edge portion of an image formed on the photosensitive member to a nonimage portion, thereby suppressing an edge effect, from the viewpoint of improvement in image quality such as reproducibility of a solid portion having a uniform density or reproducibility of a fine line. Further, while a magnetic brush composed of carrier and toner is formed in development, it is necessary to reduce a scavenging force (a force of scavenging the photosensitive member) by the magnetic brush from the viewpoint of eliminating scavenging lines generated at the solid portion by the magnetic brush to perform uniform development. To meet this requirement, there is a tendency to use a spherical carrier having a small particle size of 50 .mu.m, for example, as compared with a conventional carrier having a particle size of 100 .mu.m.
The combination of such a strong development field and a small-sized carrier can provide a high-quality image. However, a magnetic deposition force of the small-sized carrier is extremely smaller than that of the conventional carrier having a particle size of about 100 .mu.m, and the strong development field has an adverse effect to cause a problem that the small-sized carrier is easily deposited to a nonimage portion of the photosensitive member by an electric field. As mentioned above, if the carrier is deposited onto the photosensitive member, the toner present in the periphery of the carrier on the photosensitive member cannot enough come into contact with a paper in a transfer step, causing transfer defect to remarkably reduce the image quality.
As mentioned above, various devices for collecting the carrier deposited onto the photosensitive member have conventionally been proposed. To collect the small-sized carrier deposited onto the photosensitive member, it is necessary to apply a magnetic field or an electric field stronger than that in the case of collecting the conventional large-sized carrier. The magnetic field or the electric field can be enhanced by locating the collecting device near the photosensitive member. However, mere location of the collecting device near the photosensitive member causes a problem that the carrier collected by the collecting device may rub against an image portion of the photosensitive member during feeding the carrier to generate while lines.